1. Field of the Invention
The present invention relates to a method for cleaning a surface of a semiconductor substrate such as silicon wafer.
2. Description of the Related Art
In this type of the semiconductor substrate, there are attached such as metal impurities, particles having a diameter smaller than 1 .mu.m, and organic substances, onto the surface of the semiconductor substrate, and there are also formed working damages therein. As the degrees of integration and performance of semiconductor devices have been further increased, it has been increasingly demanded that the surfaces of semiconductor substrates are not contaminated such as by these metal impurities, particles and organic substances, and that the working damages are further decreased. Thus, the cleaning technique of the semiconductor substrate tends to have become extremely important among the semiconductor devices techniques.
As a conventional cleaning method of the semiconductor substrate, there is known an RCA cleaning method adopting an SC-1 solution including hydrogen peroxide solution and ammonium hydroxide, and an SC-2 solution including hydrogen peroxide solution and diluted hydrochloric acid. In this RCA cleaning method, the semiconductor substrate is firstly immersed in the SC-1 solution to thereby remove particles and organic substances from the substrate, by means of nature of oxidizability and alkalinity of the solution. Namely, within this SC-1 solution, there will simultaneously occur both of oxidation and reductive reactions, in which reduction by ammonia and oxidation by hydrogen peroxide occur competingly, and at the same time, particles and organic substances are desorbed from a surface of the substrate by etching effect due to ammonium hydroxide solution. Further, there are removed fine mechanical damages caused by working of the semiconductor substrate. Thereafter, the semiconductor substrate is immersed into an aqueous hydrofluoric acid solution to thereby remove a natural oxidized film, followed by immersion of the semiconductor substrate into the SC-2 solution to thereby remove alkali ion and metal impurities insoluble in the SC-1 solution.
As such, in RCA cleaning, the surface of the substrate cleaned by etching effect of ammonium hydroxide solution is re-cleaned by cleaning the substrate by the acidic solution.
Meanwhile, there have been disclosed a cleaning solution and a cleaning method of a semiconductor substrate, adapted to prevent such as; adhesion of metal impurities in a cleaning solution onto a surface of the substrate during cleaning of the semiconductor substrate; and re-adhesion of the metal impurities onto the surface of the substrate which have been once removed therefrom; to thereby restrict growth of an unnecessary natural oxidized film (Japanese Patent Application Laid-Open No. HEI-7-94458(94458/1995)). This solution comprises an acidic solution such as hydrofluoric acid, including from 0.0001 to 0.001 wt. % of ammonia or from 0.0005 to 0.01 wt. % of ethylene diamine tetraacetic acid (EDTA). This cleaning solution is to remove an oxide film, so that the pH of the solution is kept at about 1 corresponding to strong acid. According to this cleaning method, when a semiconductor substrate is immersed into the cleaning solution, there are simultaneously progressed removal of a natural oxidized film and removal of metal impurities, so that the metallic elements transferred into the cleaning solution will form complexes, i.e., metallic complex salts, and thus the metallic element are masked. The surface of the semiconductor substrate is negatively charged in the acidic solution, while the metallic complex salt forms complex ion in the acidic solution so that the surface of metallic complex salt is also negatively charged, resulting in prevention of re-adhesion of the metallic complex salt, i.e., of the metallic element, onto the substrate.
However, the aforementioned RCA cleaning method is problematic in that two actions of oxidation and reduction occur competingly in one vessel. Firstly, those metal impurities liberated from the surface of the substrate will remain in the SC-1 solution, and may re-adhere onto the surface of the substrate due to the surface potential of the impurities. Secondly, even if it is intended to form a metallic complex salt by complexing the metal ion in the SC-1 solution making use of an organic acid, the organic acid is oxidation-reduction treated and thereby decomposed in the SC-1 solution, resulting in considerable decrease of the complexing effect of the organic acid. Thus, the RCA cleaning method has such a defect that the metal impurities may not be sufficiently removed, depending on the sort of metal.
Further, in the cleaning method disclosed in the aforementioned Japanese Patent Application Laid-Open No. HEI-7-94458, the surfaces of particles are slightly positively charged in the cleaning solution of strong acid. Thus, it is problematic that the particles can be hardly removed since they heteroaggregate onto the surface of the substrate which has been positively charged from 0 (zero), even if the metal impurities can be removed from the semiconductor substrate based on the aforementioned reason.